Different types of tools are used to perform hundreds of processing operations during semiconductor device fabrication. Most of these operations are performed in process chambers at very low pressure, i.e., in a vacuum or partial vacuum. Such process chambers may be arranged about a central hub, and the hub and process chambers may be kept at substantially the same very low pressure. Wafers may be introduced to the process chambers by wafer handling systems that are mechanically coupled to the process chambers and/or central hub. The wafer handling systems transfer wafers from the factory floor to the process chamber. The wafer handling systems may include load locks to bring the wafers from atmospheric conditions to very low pressure conditions and back, and robots to transfer the wafers to various positions. Wafer handling systems may utilize robots that operate outside of the vacuum environment, e.g., robots that operate in the ambient factory floor environment, and robots that operate within the very low pressure environment of the process chambers and central hub. Throughput—the number of wafers processed in a period of time—is affected by the process time, the number of wafers that are processed at a time, and the time it takes to introduce the wafers into the vacuum process chambers.